Strange Earth Leakage issue

[Pieter00]

Hi Fellow Electricians, I need some advice with an issue I've never encountered before.
The customer is building an addition to his out building requested that the sub db have it's own earth leakage and taken off from the earth leakage in the main db at the house. So we did this. This sub db has 5 plugs and 3 lights and a swimming pool db. the lights are not on earth leakage. so this is where it gets interesting. When we switched the power on the lights, plugs and pool was working fine until a couple minutes later, the earth leakage in the sub db tripped. The time it takes to trip varies and when I Meggered each circuit between live/neutral/earth, I did not pick up any fault. It sometimes trippes after 2 minutes and sometimes after 20minutes. It seems to happen when the pool is running as nothing else is plugged in. But when I bypassed the earth leakage in the sub db and put the supply cable to the sub db back on to earth leakage at the main db and funny enough, it never tripped out again. the house earth leakage trippes on 20mA with the plug tester and the one I had in the Sub DB tripped at 30mA and I also changed the earth leakage twice. Any Advice on why it trippes at the sub db's earth leakage and not on the earth leakage at the main db in the house?

[Justloadit]

A neutral connection problem?

[Pieter00]

How so?

[Justloadit]

If the Neutral is not correctly wired in the secondary DB, any current flow will be unbalanced and the ELB will trip.
Alternatively there is a Neutral to earth fault some where, which will also cause an earth fault. This fault could be due to humidity or insects

[AndyD]

A crossed nuetral will usually cause an immediate RCD trip. With a fault that takes a few minutes to trip you're better monitoring the leakage over time with an earth leakage clamp meter clamped around the L+N simultaneously. This has the advantage of letting you run everything and monitor the leakage under normal operating conditions rather than the megger which gives a result whilst nothing is running.

[Pieter00]

That's an interesting point. I googled this tester and will get it and try it out. Thanx

[gobbleteller]

The customer is building an addition to his out building requested that the sub db have it's own earth leakage and taken off from the earth leakage in the main db at the house.

Just a quick question with the above statement... Is the sub db's supply cable taken from above or below the main db's earth leakage?

[Pieter00]

It was taken above the earth leakage of the main DB and was put back under the earth leakage in the main db and then the earth leakage stopped tripping

[bergie]

i also think its an incorrect neutral connection in the sub db. when you reconnected the sub db to the earth leakage relay in the main db and bypassed it in the sub db,then there is no more neutral fault in the sub db as both neutral bars (in the sub db) become one.
also i have found it when i have made that same mistake i have tested with the plug tester ,the earth leakage relay trips fine,but once you plug in a load the e/l trips.
willing to wager

[Pieter00]

The supply cable is not on earth leakage at the main db and also not on the sub db. If there was a neutral fault on the circuit the earth leakage would trip as soon as it carries load. It's not tripping as soon as there's load but trips in a couple of minutes, always different times. A neutral fault does operate like this

[Slow Blow]

Never feed an ELU from an ELU

[BlueArc]

was this problem ever resolved? I would have liked to see what the outcome was.

[DieterT]

30mA ELU feeds from 300mA ELU / 500mA ELU (hospital,agricultural environments)
But besides this, the earth leakage monitoring is one of the best ways to see what is the cause of the problem, but there are less expensive alternatives.

For one it is good to understand what operates the earth leakage unit. The reason it could be tripping is due to an imbalanced neutral (on single phase systems)
Another reason is like what was mentioned earlier, is mixed neutrals (i.e. a neutral ring circuit, one side from one E/L other side from other E/L) or crossed neutrals where only when load is applies it trips.

Possible reason that the house E/L isn't tripping then bypassing the SubDB E/L and reconnecting the supply to the MainDB E/L is because of the distance run and proper earthing at the MainDB. Therefor fault dissipates better there for not laying in the circuit causing the E/L to trip.

Step one, would be to make sure that the neutral feeding the SubDB is fed straight from the supply neutral bar. Make sure that your earth resistance is not to high and that there isn't a volt drop on the supply due to long runs.

Step two, would be like you have done to megger each supply. Be careful when meggering between live and neutral, you could pop your tester and what ever is on the other side

You can install an additional E/L unit and separate the circuits feeding under each one, try and isolate the problem. Remember, it can also be to accumulation of leakage to earth, such as what you get when a bunch of desktop computers are put together under one E/L unit. It causes it to trip due to that each power supply leaks a little bit of power to earth.

Isn't finding an earth leakage fault alot easier when it is constant?
If you say it seems to be when the pool pump runs, try disconnecting the pool pump and see what happens. Alot of times when a pool pump gets hot, it expands and water can actually leak into the router and cause an earth fault. If it stops tripping, disconnect the motor windings from the capacitor and remove the star/delta connection then megger each winding beginning/end to earth to see if there is some leakage to earth.